The present invention relates to a method of producing a crystalline semiconductor material by heating an amorphous semiconductor material or a polycrystalline semiconductor material so as to crystallize the material, and a method of fabricating a semiconductor device using such a crystalline semiconductor material.
In recent years, semiconductor devices used, for example, for a solar cell including such semiconductor devices formed on a substrate in an array pattern and for a liquid crystal display unit including such semiconductor devices as pixel drive transistors have been actively studied and developed. Further, in recent years, to realize a high degree of integration and multi-function of semiconductor devices, a three-dimensional integrated circuit including the semiconductor devices stacked on a substrate has been actively studied and developed.
A glass material such as artificial quartz or a plastic material has drawn attention as a substrate material used for these semiconductor devices because the glass material or plastic material is inexpensive and is easily formable into a large-size substrate. In general, when a semiconductor thin film is deposited on a substrate made from such an amorphous insulating material, since the amorphous insulating material has no long-range order, the deposited semiconductor thin film has an amorphous or polycrystalline structure.
For example, with respect to a thin film transistor (TFT) used as a pixel drive transistor in a liquid crystal display unit, an operational region (channel region) is formed by a polycrystalline silicon (Si) film formed on the above-described substrate. The use of the polycrystalline silicon film for forming the operational region, however, has a disadvantage that the crystallinity of the polycrystalline silicon film is poor because crystal grains boundaries are present at random in the fine structure of the film and crystal grains have different face orientations. Another disadvantage is that as the grain sizes of crystal grains in the polycrystalline silicon film become as large as being close to the channel length of the TFT, the characteristics of the TFT may become uneven. In this way, a semiconductor device such as a TFT using a polycrystalline silicon film is very inferior in characteristics to a semiconductor device using a single crystal silicon film.
From this viewpoint, a single crystallization technique of a silicon film formed on a substrate made from a glass material has been proposed. For example, an attempt has been made to form a single crystal silicon film on a substrate made from silicon oxide by using a ZMR (Zone Melting Re-crystallization) technique (see H. A. Atwater et al.: Appl. Phys. Lett. 41 (1982) 747, or K. Egami et al.: Appl. Phys. Lett. 44 (1984) 962). Another attempt has been made to form a silicon film having a very large area on a substrate made from quartz or glass (see A. Hara et al.: AMLCD Technical Digest p. 227, Tokyo 2002).
The ZMR technique allows formation of a silicon film having a large area, but has a difficulty in control of orientation of crystal grains and crystal grain boundaries. Accordingly, a silicon film formed by the ZMR technique contains crystal grain boundaries that are present at random, and is therefore difficult to be applied to three-dimensional integration of semiconductor devices, which integration requires a high-level equalization of the semiconductor devices. Another problem of the ZMR technique is that since the ZMR technique is a high temperature process requiring a large thermal load such as about 1450° C., such a ZMR technique cannot be applied to a plastic material expected as a substrate material. Taking into account the heat resistance of a plastic material, a low temperature process performed at about 200° C. or less is desirable.
In recent year, a method of producing a silicon film having crystal grains grown in the {111} orientations on a buffer layer made from silicon nitride by laser irradiation using a second harmonic neodymium laser (Nd:YVO4 laser) having a wavelength of 532 nm has been disposed (see M. Nerding et al.: Thin Solid Films 383 (2001) 110). This method, however, has an inconvenience that since a silicon film having crystal grains grown in the {100} orientations has been used for a semiconductor device in an advanced MOS transistor, the silicon film having the crystal grains grown in the {111} orientations cannot be applied to a process of fabricating the above-described MOS transistor. In addition, the reason why the silicon film having crystal grains grown in the {100} orientations is used for the above-described semiconductor device is that the silicon crystal grains having the {100} orientations are lower in interface level density than silicon crystal grains having face orientation other than the {100} orientations, and therefore, suitable for forming a transistor sensitive to interface characteristics.
As described above, according to the related art techniques, it has been difficult to control crystal grain boundaries of a crystalline film formed on a substrate made from a glass material or plastic material, to control the face orientations of crystal grains to specific face orientations (for example, {100} orientations for a silicon film) with respect to the vertical direction of the substrate, and to control face orientations of the crystal grains in the in-plane direction of the substrate, thereby failing to equalize the quality of a semiconductor device and sufficiently enhance the performance thereof.